Whisk together the powdered sugar, corn syrup, milk and vanilla in a bowl. Whisk until the mixture is smooth and glossy. It should be thick if you are using it as a piping. Add a 1/2 teaspoon more milk at a time to make the filling consistency. Tint the icing with food color, if desired.
Spread onto the cookies using a knife, spoon or frosting bag or bottle.
Decorate cookies as desired. Allow icing to harden before storing.
Make Ahead: You can easily make this icing ahead of time to frost your cookies later. Simply prepare as directed, then store in the fridge in an airtight container or zip top bag with no air leaking in. Set out about 30 minutes before using. Drying tip: It helps to dry the cookies in front of a heater fan to keep the icing shiny.Storage: Once the icing has hardened the cookies can be stacked. I like to put a piece of parchment paper in between. Store in a covered container for 3 days or in the freezer for up to 3 months.